In Japanese speech, you will encounter words, such as DESU/MASU (です、ます) where the SU is omitted - or very faint. However, in writing, it must have the す. When writing in Hiragana, you must write each syllable, whether it is silent when said out aloud or not.
It is like in English, when you say "wanna" for "want to" or "gonna" for "going to". If you were to write our these phrases, in a report, for example, you don't want to write "wanna" or "gonna" (unless in extremely colloquial writings).
Hope that helps!